On your server, connect to the internet and run the following two commands:
ifconfig will tell you your IP address that you received from the DHCP server, your subnet mask.
route -n will tell you what your gateway's IP address is (gateway being the router that you are connected to).
Hopefully you are connected to a router and not directly to the internet - because when you are connected directly to an ISP, you can't just randomly assign yourself an IP address - they have to assign you one. If you have purchased a static IP address form the ISP, then you can use that. Otherwise, your router will receive a DHCP IP address from the ISP and, since you probably use NAT, you probably have an IP address such as 192.168.1.11 or similar with a subnet mask of 255.255.255.0.
If you do have the 192 address (which is a private address that gets translated by the router into a public address when it passes through), then set your server up with a static ip address on the same network - like this:
Gateway: 192.168.1.1 (or 192.168.1.254 - whichever it is)
Next, you have to go into your router and set up port forwarding; traffic heading to your webserver is looking for port 80. So, port 80 gets forwarded to 192.168.1.100
Now, the most annoying part is, if you don't have a static IP address assigned by your ISP, then you will need to use Dynamic DNS. The easiest way to do this is hard to say. For me, I have a Linksys WRT54GS router and I have replaced the Linksys software with DD-WRT. DD-WRT has a DDNS (Dynamic DNS) section in its web interface that allows you to point to a DDNS host with your username and password. I use freedns.afraid.org which is free and easy to set up.
Let me know if I am even in the ballpark as far as the info you are looking for please.